1. Field of the Invention
The present invention relates to a mold material for use in molding a titanium cast to be mounted into the mouth as a dental prosthesis.
2. Description of the Prior Art
Generally, the titanium is light in weight and superior in corrosion resistance and also in biocompatibility. But it has drawbacks such as tendencies to be easily oxidized at a high temperature as well as to be easily contaminated within a crucible and within a mold. Therefore, in recent years there have been proposed several kinds of mold materials for use in casting the pure titanium or titanium alloys.
As a mold material for use in molding a dental titanium cast, so far have been made known a material having magnesia for its main ingredient, a material having zirconia for its main ingredient, a material having calcia for its main ingredient and the like by a scientific essay or a scientific lecture.
Generally, the dental cast is formed as a thin cast which is not more than several millimeters in thickness and is often ground by a dentist for a fine adjustment in order to fit the cast in the mouth. Therefore, in case that there exist inner blowholes in the cast, the blowholes appear in the external surface of the cast due to such grinding. Accordingly, it is required to mold the dental cast having no defects and also to provide a smooth external surface for the cast.
Further, since the dental cast is mounted into the mouth, the cast often comes into contact with the mucosa of the mouth having the sharp sense of touch. Therefore, the feeling of mounting is greatly affected by a surface roughness of the cast. In case that there exist the surface defects and the inner blowholes appeared in the external surface of the dental cast by the grinding, the cast is accompanied with its own proper problem that the surface defects or the defect portions appearing in the surface tend to easily provide a nest for the propagation of bacteria, which problem is not found in any other casting.
Furthermore, also an adaptability of a cast for its mold, namely a high dimensional accuracy is required for the dental cast.
But, in a conventional mold material for casting the titanium there are several practical problems that a sintering is caused between the titanium and the mold material to provide a sintering of casting surface, casting defects such as blowholes are produced and a size of the cast is reduced. Resultantly, the yield rate and the quality of the cast become worse and the cost of the titanium cast becomes higher. So far there has not been provided such a mold material as enabling to solve such proper problems of the dental cast.
In the mold material having the magnesia for its main ingredient, there are practical problems that it takes a long time for curing, the fresh mold before sintering is weak in strength and so on. In the mold made of the material having the zirconia for its main ingredient, an agar-agar usually used as an impression material doesn't cure, but only a special impression material such as a silicon rubber and a vinyl silicone can cure. Thereupon, there are also several practical problems that it takes a long time for curing, both the fresh mold and the sintered mold are weak in strength, a ringless casting is impossible, a shrinkage during the curing is large, a special ring is needed for casting and the cost is increased by use of the expensive zirconia as the main ingredient. In the mold material having the calcia as its main ingredient, there are several practical problems that a special preserving method, for example a vacuum wrapping is required for the mold because the calcia is hygroscopic and water-absorptive, a strong smell of methanol makes the working environment and hygiene worse because the calcia mold material can not be kneaded well with the water and should be kneaded with the methanol and the cast can not be made with a high dimensional accuracy because the calcia is subject to a curing shrinkage and is lack of expansivity.